This invention relates to a signal transmission device and a signal reception device communicable with the signal transmission device and to a communication method of carrying out communication between the signal transmission device and the signal reception device.
In a conventional communication method of the type described, a voice signal is often subjected to adaptive delta modulation (often abbreviated to ADM) to be transmitted from a signal transmission device to a signal reception device in the form of a sequence of digital voice signals which may be called an ADM digital signal sequence. It is known in the art that a sequence of information signals can be transmitted in combination with such a digital voice signal sequence.
As a rule, the information signal sequence is transmitted in the form of a non-return-to-zero signal from the signal transmission device to the signal reception device. In this event, the information signal sequence must be distinguished from the digital voice signal sequence in the signal reception device. Otherwise, the information signal sequence might objectionably audibly be reproduced together with the digital voice signal sequence in the signal reception device. Such an audible reproduction of the information signal sequence becomes noisy for listeners of the voice signal and might sound like a beep or a buzz. As a result, the listeners may become uncomfortable or uneasy.
Exact distinction of the information signal sequence from the digital voice signal sequence tends to bring about an objectionable increase of an error detection rate in the signal reception device because the digital voice signal sequence is often wrongly detected as the information signal sequence.